Charge transfer dynamics in bridged donor-acceptor molecular assemblies

El-Dali, Abdelmeneim (2011) Charge transfer dynamics in bridged donor-acceptor molecular assemblies. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The metabolic processes inherent to life on earth are driven by chemical transformations that are too numerous to count. Important classes of reactions involve the transfer of charge, i.e. electron transfer and proton transfer as well as group transfer reactions. The recent X-ray crystal structure revealed that proton coupled electron transfer reactivity plays crucial rule in the photosynthetic apparatus. The theory to describe proton coupled electron transfer is not as evolved to that which described electron transfer. There exist a number of unresolved issues. The primary goal of this work was to understand at a very fundamental level, the molecular properties that dictate chemical reactivity so as to quantify the important factors which influence the charge transfer excited-state dynamics. In this work, we have applied a variety of different techniques to study the properties and the mechanism of ground state and excited state intermolecular and intramolecular proton transfer and charge transfer in chromosphere assemblies. -- The bbim system was studied to outline fundamental investigation with respect to the excited state intramolecular proton transfer. The ground state potential energy surface was characterized by temperature dependent ¹H NMR to establish the energetics of the enol / keto inter-conversion in the ground state. Analysis of the temperature dependent ¹H NMR yielded ΔHǂ = 27 kJmol⁻¹, ΔSǂ = -133 JK⁻¹mol⁻¹. Upon excitation, bbim is a dual emitter with high energy emission band at 427 nm and 550 nm respectively. The intensity of transitions was found to be solvent dependent. Addition of D₂O resulted in dramatic changes in the intensity and energetics of emission spectrum envelope. The emission spectra were subjected to a Franck-Condon line shape analysis and provide structural and electronic of the excited states. -- The photophysical properties of bbim were dependent on the electrolyte. Dramatic changes in the absorption and emission spectra were observed with [NBu₄]F and [NBu₄]OH. Titration experiment with a number of [N(Bu)₄]X, where X is fluoride (F⁻), chloride (Cl⁻), bromide (Br⁻), iodide (l⁻), acetate ( Ac⁻), hydrogen sulphate (HSO₄⁻), hexafluorophosphate (PF₆⁻). The equilibrium constants were valuated using singular value decomposition, or global analysis. The analysis suggests a complex equilibrium consisting of eight distinct species in the ground stat. For the excited state bbim a similar analysis describe above yielded six step equilibra. The potential application of bbim as a anion sensor which can discriminate the anion however this compound are air sensitive when excited in present of air. -- A second distinct study was focused on the 3-aminocoumarin (3-AC). The emission spectral fitting results were consistent with a small change in the structural in the ground and excited state. This study was first to use emission spectral fitting as a tool to probe the excited state and was consistent with expectation of the energy gap law was obeyed. At the present time more experiments are required to understand the position of substituent and the nature of emitting state. The data suggests the intervention of a ICT and TICT state in excited state decay. DFT calculations were carried out to understand the electronic structure in the ground state.

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